Method of improving the piezoelectric properties of ceramics

ABSTRACT

AND BI(L1/0M1/2)O3-PBZRO3-PBTIO3 (WHERE M IS EITHER NB, TA OR SB) ARE IMPROVED BY THERMALLY DIFFUSING AVERAGE AMOUNT BY WEIGHT OF ABOUT 0.05 TO 0.5% CR2O3 THEREIN.   PB(NI1/3NB2/3)-PBZRO3-PBTIO3   (WHERE M IS EITHER NB OR SB),   PB(FE1/3NB2/3)-PBZRO3-PBTIO3   (WHERE M IS EITHER NB, TA OR SB),   PB(LI1/4M3/4)O3-PBSRO3-PBTIO3   PIEXOELECTRIC PROPERTIES OF CERAMICS OF THE TYPE

United States Patent 3,718,595 METHUD 0F IMPROVING THE PIEZOELECTRICPROPERTIES OF CERAMICS Sadayuki Takahashi and Norio Tsubouchi, Tokyo,Japan, assignors to Nippon Electric Co., Ltd., Minato-kn, Tokyo, JapanNo Drawing. Filed Mar. 20, 1972, Ser. No. 236,491 Int. Cl. C04b 35/46,35/48 US. Cl. 252-62.9 14 Claims ABSTRACT OF THE DISCLOSUREPiezoelectric properties of ceramics of the type Pb (Li M O -PbZrO-PbTiO (where M is either Nb, Ta or Sb),

' Pb(Fe Nb )-PbZrO -PbTi0 (where M is either Nb or Sb),

Pb (Ni Nb O -PbZrO -PbTi0 and Bi(Li M )O -PbZrO -PbTiO (where M iseither Nb, Ta or Sb) are improved by thermally diffusing Cr in saidceramics whereby to provide an average amount by weight of about 0.05 to0.5% Cr 0 therein.

This invention relates generally to a method of obtaining piezoelectricceramics in which mechanical strength against a bending force is large,the mechanical quality factor (Qm) is small, and the electromechanicalcoupling coefiicient (Kr) is large. Such ceramics are suitable for useas piezoelectric elements in wideband Wave filters.

The valueof Kr of piezoelectric element is an important factor whendetermining the bandpass width of the filter. The bandpass width can beincreased by increasing Kr, but to a limited value. Namely, the value ofKr'cannot be larger than 1. For this reason, a ceramic filter or amechanical filter using a piezoelectric element as its transducer hashitherto been limited in use over a relatively narow hand. For Widebandapplications, it is the practice to use an LC filter.

Since the recent trend has been to reduce markedly the size ofcommunications equipment, the need for smaller size filters has becomequite apparent. In this connection, it has become essential to providemechanical or ceramic filters for use in wideband applications. Underthe circumstances, the piezoelectric element is expected to have areduced Qm value. The specific bandwidth (B) of a ceramic filter isexpresed by the following equation:

(where 'y is the capacitance ratio).

The capacitance ratio 7 is dependent on the value of Kr. The larger thevalue of Kr, the smaller becomes the value of 'y. Practically speaking,the Kr of the piezoelectric element has already reached its maximumavailable value, and it is difficult to realize a smaller capacitanceratio '7; whereas, there is no limitation as to smaller values of Qm.Hence, to obtain a wideband filter having an increased specific bandwith(B), it is necessary to reduce the value of Qm.

Ceramic compositions of PbZrO -PbTiO Pb(Mg Nb )O -PbZrO -PbTiO and Pb(Ni Nb )O -PbZrO-PbTiO systems, as well as ceramic compositions of thesame systems as these with certain additive elements, exhibit large Krvalues above 50%. On the other hand, there are difficulties insatisfactorily reducing the value of Qm. For example, when 3,718,505Patented Feb. 27, 1973 Qm is smaller than 80, the value of Kr isaccordingly small.

While PbNb O is known as a compound whose Qm is as small as about 10;its Kr value, on the other hand, is too small and this compound also isimpractical for use as a wideband filter element.

It has previously been found by one of the inventors herein that thevalue of Qm can be reduced by injecting or incorporating an impurityinto the PbZrO -PbTiO system ceramics by thermal diffusion followingformation of the system. (Ref: M. Takahashi and S. Takahashi; Japan J.Appl. Phys. 9 (1970) (1006.)

Table 1 below, for example, shows by way of comparison the results of aset of experiments on a ceramic body expressed in terms of Pb (Zr Ti )0Some of the ceramic samples were produced by adding CR O as one of theraw materials; while other samples contained Cr O which was injected orincorporated therein after production of the ceramics.

ice

TABLE 1 Dielectric constant (e) No impurity Additive of 0.05 weightpercent Orzo: Injection of 0.05 weight percent CrzOa Additive of 0.3weight percent C1203 Injection oi 0.3 weight percent CF20;

As shown in Table 1, the value of Qm can be reduced by injecting orincorporating Cr O into the system ceramics after the system isproduced. At the same time, however, the value of Kr is also reduced.Namely, this ceramics is not suited for wideband filter element.

THE INVENTION The ceramic materials improved according to this inventionare highly practical for use as the piezoelectric element forwidebandfilters; with the Kr more than 50% and with the Qm factor less than 30.

The present invention uses, as base ceramics,

and Bi(Li M )O -PbZrO -PbTiO (where M is either Nb, Ta or Sb), in whichsystems part of Pb (up to 15%) may be replaced by Sr, Ba or Ca. As theaccessory ingredient, 0.05 to 0.5 weight percent Cr O is used, which isincorporated into the base ceramics by thermal diffusion.

The base ceramics used in this invention may be produced by well knownmethods. By way of example, they are produced by weighing and mixing thestarting materials, calcining the mixture at about 700 to 1000" C. in anoxidizing or inert atmosphere such as in air or nitrogen for a time morethan 30 minutes, crushing the calcined mixture, forming the mixture intoa desired shape under a pressure of more than about kg./cm. sinteringthe mixture at about 1000 to 1300 C. in an oxidizing or inert atmospherefor about 30 minutes to 10 hours, and then cooling the sintered body toroom temperature. In this process, the calcining may be omitted.

Chromium can be thermally diffused into the base ceramics either from asolid phase or a vapor phase. Typically, Cr O can be used as a solidsource material of the Cr diffusion. In one example, a paste consistingof Cr O powder and a binder is applied to the surface of the baseceramic body. Alternatively, the base ceramic body is buried in a powderof Cr O or in a powder of Cr O and A1 or sandwiched between two Cr Oplates. Whatever the form, the base ceramic body and Cr O powder orplate is heated at about 900 to 1300 C. in an oxidizing or inertatmosphere to effect the thermal diffusion of Cr into the base ceramicbody. It is believed that the oxygen atoms are at the same time diffusedinto the ceramic body whereby Cr O is finally incorporated in theceramic body. The source of diffused oxygen atoms may be Cr O and/ orthe atmosphere if the oxidizing atmosphere is used. Thermal diffusion ofCr from vapor phase may be carried out, for example, by use of CrCl invapor form at about 800 to 1300" C. in an atmosphere containing oxygen.

The diffusing process should be continued until the desired averageamount of Cr O is obtained in the base ceramic body, that is, until theCr O in the ceramics reaches a value of 0.05 to 0.5% by weight of thebase ceramics. When an amount of the source of Cr diffusion iscontrolled to a limited value and the thermal diffusion is continued fora sufficient period of time, Cr O is uniformly contained everywhere inthe base ceramic body, otherwise, the Cr O content tends to be locallyhigh at the part of the base ceramic body near the surface thereof andlow at the part remote from the surface. In this case, it is preferredthat the local content of Cr O be more than 0.001 weight percent at thepart where the C1' O content is lowest and less than 0.5 weight percentat the most dense part.

In order to reduce the size of the filter, the size of the piezoelectricelement must be reasonably small. Piezoelectric elements thinner than0.1 mm. are often required. Since the mechanical strength againstbending (or the deflective strength) of the ceramic materials can bemore than 1500 kg./cm. according to this invention, the ceramicmaterials are unbreakable when they are shaped into a thin plate.

The improved ceramic materials produced according to this invention arehighly suitable for use as small size, wideband filter elements and arecapable of a broad range of applications.

DETAILS OF THE INVENTION As illustrative of this invention, thefollowing examples are given:

Sample No.

Example 1 Unless otherwise stated, the starting materials of the basematerials used were lead monoxide (PbO), calcium carbonate (CaCstrontium carbonate (SrCO barium carbonate (BaCO lithium carbonate (LiCO 5 9. 9. 9. 00c mummies-ami e 4 niobuim oxide (Nb O tantalum oxide (TaO and antimony oxide (Sb O in powder form. Chromic oxide was thermallydiffused into the base ceramics produced from the foregoing materials.

The powders were weighed in accordance to the composition desired andmixed together with pure Water or alcohol. The mixed powder was driedand calcined at a temperature of about 700 to 900 C. in air for onehour. The sample was crushed and a small amount of water added followingwhich the crushed material was pressed at 700 kg./crn. and formed into adisc with a diameter of 20 mm., then sintered at about 1100 to 1300 C.in air for one hour, and cooled to room temperature. The Cr O powder waskneaded with a small amount of water and applied to the surface of thesample ceramics. For injection of Cr O thereinto by thermal diffusion,the samples were subjected to heat treatment at about 1000 to 1200 C. inair for 0.5 to 2 hours. The amount of Cr O diffused therein was measuredby the extinction light intensity method. Part of the sample ceramicswas used for the purpose of measuring the mechanical strength againstbending (hereinafter referred to simply as mechanical strength). Themeasurement was based on the method disclosed in Relation BetweenSintering Temperature and Structural Characteristics of Ceramics forCircuit Elements by Ichiro Ida et al. [Electrical CommunicationLaboratory, Nippon Telephone and Telegraph Corporation, ResearchApplication Report Vol. 16, N0. 6 (1967), pp. 1l49ll74]. The rest of thesample ceramics was polished to a thickness of 1 mm. Silver electrodeswere attached to the samples by baking, and a DC field of 5 kv./mm. wasapplied to the samples at C. for one hour as a poling treatment.

After storing the samples for more than 24 hours, the Kr and Qm factorswere measured. Unless otherwise stated, the measured results hereinaftershown are derived from samples produced and treated in the same manneras described for Example 1.

Table 2 shows the measured results of Kr, Qm and the mechanical strength(deflective strength) of the samples obtained from ceramics expressed bythe formula I-t t) 1/4 3 4)u 'v w 3 (where M is either Nb, Sb, or Ta,and M is either Ca,

Sr, or Ba; u+v+w=1), into which Cr O is incorporated by thermaldiffusion.

TABLE 2 Amount of CIZO; incorporated (1: wt. percent) Mechanicalstrength (kg/cmfl) Kr (percent) none of these containing Cr -O This noteis similarly applied to the succeeding examples. From Table 2, it isapparent that Kr is markedly large and Qm is as small as 85 for theasterisked samples.

Example 2 Table 3 shows the results of experiments on solid solutionceramics expressed by the formula However, when Cr O is incorporated bythermal dif- 5 L-t' t') 1/3 2/3)u v' w'] 3 fus1on 1nto the ceramics, Qmis considerably reduced with where is Ca Sr or Ba, and M is Nb, or anl11:1ChaI lged Increased Value of a (u'+v'+w'-=1) into which Cr O wasincorporated by It 1s quite apparent that the mechanical strength isconthermal diff i y incfeased when 5 is incorporated y In thisembodiment, Fe O is used in terms of weight thermal diffusion (noteSamples 1.10 and 1.18). of F 0 as th starting aterial of Fe,

TABLE 3 Amount of Crtzt'z a (lncog K Mecthanlcfixl 01 e Z W 1' S [ellSample No M i M u 0' w p a percent) (percent) Qm (kg/c115 2.1 Nb 0.01 0.51 0.43 0 52 2.2... Nb 0.01 0. 51 0.48 0.3 52 2.51- Nb 0.20 0.47 0. 33 054 2.4. Nb 0.20 0.47 0. as 0.25 55 2.5 Nb 0. 0. 42 0. as 0 58 2.6. Nb 0.20 0. 42 0. 3s 0. 59 2.7 Nb 0. 05 0.52 0.43 0 65 2.8. Nb 0. 05 0.52 0.43 0.5 66 20* Nb 0.05 0.40 0.46 0 68 2.10 Nb 0.05 0.40 0.46 0.3 68 2.11Sb 0. 05 0.52 0.43 0 65 2.12 sb 0. 05 0. 52 0. 45 0. 05 65 2.13 sb 0. 020. 51 0. 47 0 77 2.14 Sb 0.02 0.51 0.47 0.1 70 2.15 0.1 sb 0. 02 0. 510.47 0 75 2.16 0.1 sb 0. 02 0.51 0.47 0.2 70 2.17 0.1 sb 0. 02 0. 510.47 0 71 2.1a 0.1 Sb 0. 02 0. 51 0.47 0.1 75 2.10 0.1 Sb 0. 02 0. 510.47 0 04 2.20 0.1 sb 0. 02 0. 51 0.47 0.15 04 The above resultsindicate that the compound of this If Cr Og, is not incorporated in theceramics by difinvention is highly practical when used as widebandfilter fusion, the value of Kr is very large. However, the valueelements. of Qm cannot be reduced to below about 90. When Cr O In thePb(L1 M )O -PbZrOyPbT1O system SOlld is incorporated, the value of Qm isremarkably reduced solution (where M may be Nb, Ta, or Sb), theeffective with an unchanged or increased value of Kr. Thus, these rangein which the value of Kr is maintained above 50% materials can be usedas an excellent wideband filter eleis given in terms of t, u, v, w and xas indicated below ment. when the ceramics are expressed by the formulaIt is apparent that the mechanical strength is consider-(PbktM'd[(Li1/4M3/4)uZrv-I-iw]O3+x glglylpilreicilegsgdlgy v1rtue of thepresence of Cr O (note Percent 2 3 (Where is Ca, Sr, of Ba, and M1 Thecompositional range where the value -of Kr is maintained above 50% isgiven below in terms of t, u,

v, w and x. (1 0 15745020 0t'0.15 0.37v0&5 0.0*1u'0.20 0 0.48 0.42v'0.520055x205 0.33w0.48 Outside this range, the value of Kr is decreased, and0.05x'().50 the effect of expanding the bandwidth of filter is small,Example 3 even if Qm is reduced. In other words, the effective rangeaccording to this invention should preferably be as speci- Solidsolution ceramics expressed by the formula .fied above. In thesucceedingexamples, the effective range 4 is defined on the same basis. It is tobe noted that the '){(N11/ 3Nb2/ amount of x of G 0 mentioned in thisand the succeed- (Where M is Ca, Sr, or Ba; u"+v+w"=1) were used, ingexamples is of an average value, that is a ratio of into which Cr O wasincorporated by'thermal dilfusion. the total amount by weight of .Cr Ofinally contained Table 4 shows the relations among the quantity of inina sample to the weight of the sample. corporated Cr O Kr, Qm andmechanical strength.

TABLE 4 Amount of GU03 incorporated Mechanical (1 wt. Kr strength 74" a"percent) (percent) Qm (kg/cm!) 0. 35 0.30 0.35 0 62 0. 35 0. 30 0. 35 0.5 62 0. 84 0.40 0.26 0 50 0. 84 0. 40 0. 26 0. 4 50 0.50 0.10 0.40 0 540. 50 0. 10 0. 40 0. 5 55 0.55 0.15 0.30 0 52 0. 55 0.15 0. 30 0. 45 550.40 0.23 0.57 0 6s 0. 40 0. 23 0. 37 0. 2 69 0.40 0. 23 0.57 0 61 0. 400. 25 0. 37 0. 25 61 0.40 0.25 0.37 0 5s 0. 40 0. 25 0. s7 0. a5 59 0.400. 25 0.57 0 58 0. 40 0. 23 0. a7 0. 3 59 Table 4 indicates the factthat in the ceramics in which the Cr O was omitted, the value of Kr isincreased and the value of Qm is decreased.

Whereas, when Cr O is incorporated thereinto by ther- 8 What is claimedis: 1. A method of improving the piezoelectric properties of ceramicsconsisting essentially of the following composition:

mal diffusion, the value of Qm is remarkably reduced 5 uPb(Li M )O-vPbZrO -wPbTiO with an unchanged or increased value of Kr. At the samewher p es n s a element seleCted from the r p time, the mechanicalstrength is considerably increased consisting of Nb, Ta and Sb and u, vand w represent a (note sample 3 d 3 1() set of mol ratios having thefollowing relation:

The compositional range where the value of Kr is main- 0.0lu0.20 tainedabove 50% is given below in terms of t", u", v", 10 0.37v0.52 w and x".0.34w0.48 0t"0.15 u+v+w=1.00 i xicomprising the step of incorporating Crinto said ceramics by thermal diffusion to the extent that said ceramicscon- 0.30w0.40 tain Cr O in an amount of about 0.05 to 0.5% by weight ofsaid ceramics.

Example 4 2. The method of claim 1, in which Cr O is used as b h f 1 asource of Cr diflfusion in said incorporation step. Sohd Solutlonceramlcs expressed y t e ormu a 3. The method of claim 1, in which saidincorporating (Pb N HBi "M3,...)[Li M HZr WTi HJO step comprisescontacting said ceramics with Cr O and then heating said ceramics and CrO at about 900 to EYP Z i or g g s fl g $2 5 1300 C. in an oxidizing orinert atmosphere for a time +v (in; E f g 1 Tabl 5 relzk sufiicient forsaid ceramics to contain Cr O in an amount ii 6 C 0 Z K of about 0.05 to0.5% by weight of said ceramics.

lens 5 3 F 0 r2 3 l C rp m 4. A method of improving the piezoelectricproperties Qm an mec amca S reng of ceramics consisting essentially ofthe following com- Example 4 position:

I Solid solution ceramics expressed by the formula u Pb(Fe1/3M2/3)O3'vPbZrOa'w PbTlOs l where M represents an element selected from the group1u"-t"' u" t"')[ l/z l/z)ii"' v"' w"'] 3 consisting of Nb and Sb, and u,v and w represent a set (where M is Nb Sb or Ta, and r is Ca, Sr or ofmol ratios having the following relation: u'+v'+w"'=l) were used, intowhich Cr O was in- 0,01 '0,2 corporated by thermal diffusion. Table 5shows the rela- 0.42 '0,52 tionship among the quantity of Cr Oincorporated, Kr, 33 '0,48 Qm and mechanical strength. u+v'+w'= 1.00

TABLE 5 Amount of r20: incorporated Mechanical (1" wt. Kr strengthSample No M t" M u" v w percent) (percent) Qm (kg/cm 4.1 Nb 0. 10 0. 0.45 0 61 4.2 Nb 0.10 0. 45 0. 45 0. 35 4.3- Nb 0. i5 0. 425 o. 425 0 534.4. Nb 0.15 0. 425 0. 425 0. 4 56 4.5. Nb 0. i0 0. 45 0. 45 0 58 4.5..Nb 0. 10 0.45 0. 45 0.4 58 4.7. Nb 0.10 0.45 0. 45 0 5g 4. Nb 0. 10 0.45 0. 45 0. as 58 4, Nb 0. 10 0.45 0.45 o 54 4. Nb 0.10 0. 45 0. 45 0. 555 4, Ta 0. 10 0. 45 0. 45 0 s1 4. Ta 0. i0 0. 45 0. 45 0. 35 c2 4. Ta0. 0e 0. 47 0. 47 0 5i 4. Ta 0. 06 o. 47 0. 47 0. a 53 As shown in Table5, when no Cr O is incorporated, comprising the step of incorporating Crinto said ceramics the value of Kr is large, while the value of Qm isnot 55 by thermal difiusion to the extent that said ceramics conreducedbelow 34. Whereas, when CD 0 is incorporated tain Cr O in an amount ofabout 0.05 to 0.5% by weight by diffusion, the value of Qm is markedlyreduced with of said ceramics. an unchanged or increased value of Kr.The mechanical 5. The method of claim 4, in which said incorporatingstrength is considerably increased. step comprises contacting saidceramics with Cr O and The compositional range where the value of Kr is60 heating said ceramics and the Cr O at about 900 to 1300 maintainedabove 50% is given as follows in terms of t, C. in an oxidizing or inertatmosphere for a time sufiicient u', v', w' and x. for said ceramics tocontain Cr O in an amount of about m 0.05 to 0.5% by weight of saidceramics. 0 50.15 0 10 um 0 15 6. A method of improving thepiezoelectric properties unis-Wig gisggganmics consisting essentially ofthe following com- III 31, 38; u"Pb(Ni Nb )O -v"PbZrO -w"PbTiO where u,v" and w" represent a set of mol ratios having Although the presentinvention has been described in the following relation: gon unction withpreferred embodiments, it is to be un- I 0355,!5084 erstood thatmodifications and variations may be re- H 0.10 v 0.30 sorted to withoutdeparting from the spirit and scope of 0.30 w 0.40 the invention asthose skilled in the art will readily under- 00 stand. Suchmodifications and variations are considered to be within the purview andscope of the invention and comprising the step of incorporating Cr intosaid ceramics the appended claims.

by thermal diffusion to the extent that said ceramics contain Cr O in anamount of about 0.05 to 0.5% by weight of said ceramics.

7. The method of claim 6, in which said incorporating step comprisescontacting said ceramics with C1' O and heating said ceramics and saidCr O at about 900 to 1300 C. in an oxidizing or inert atmosphere for atime sufficient for said ceramics to contain Cr O in an amount of about0.05 to 0.5% by weight of said ceramics.

8. A method for improving the piezoelectric properties of ceramicsconsisting essentially of the following composition:

u"Bi(Li M )O -v'"PbZrO -w PbTiog where M represents an element selectedfrom the group consisting of Nb, Ta and Sb and u', v' and w' represent aset of moi ratios having the following relation:

which comprises the step of incorporating Cr into said ceramics bythermal diffusion to the extent that said ceramics contain Cr O in anamount of about 0.05 to 0.5 by weight of said ceramics.

9. The method of claim 8, in which said incorporating step comprisescontacting said ceramics with Cr O and heating said ceramics and Cr O atabout 900 to 1300 C. in an oxidizing or inert atmosphere for a timesufficient for said ceramics to contain Cr O in an amount of about 0.05to 0.5 by weight of said ceramics.

10. A method of improving the piezoelectric properties of a ceramic bodyconsisting essentially of a composition selected from the groupconsisting of ft 't) 1 4 3 4)u v w] 3 where M is selected from the groupconsisting of Nb, Ta and Sb and M is selected from the group consistingof Ca, Sr and Ba and t, u, b and w satisfy the following relation:

( i-t' 't') 1 3 2 s)u' 'v' w'] a where M is selected from the groupconsisting of Nb and Sb, and M is selected from the group consisting ofCa, Sr and Ba, and t, u, v and w satisfy the following relations:

( 1-V' 't") 1 3 2 3)u" v" w" 3 where M is selected from the groupconsisting of Ca, Sr and Ba, and t", u", v" and w" satisfy the followingrelation:

which comprises the step of thermally diffusing Cr into said ceramicbody to the extent that said ceramic body contains C12O3 in an amount of0.05 to 0.5 by weight of said ceramic body.

11. The method of claim 10, in which Cr O is used as a source of the Crdifiusion in said diffusing step.

12. The method of claim 11, in which said diffusion step comprisescontacting said ceramic body with Cr O' and heating said ceramic bodyand Cr O at about 900 to 1300 C. in an oxidizing or inert atmosphere fora time sufficient for said ceramic body to contain Cr O' in an amount ofabout 0.05 to 0.5 by weight of said ceramic body.

13. The method of claim 11, in which said diffusion step comprisescovering the surface of said ceramic body with a powder comprising Cr Oand heating said ceramic body and said powder at about 1000 to 1200 C.in air for a time sufficient for said ceramic body to contain Cr O in anamount of about 0.05 to 0.5 by weight of said ceramic body.

14. The method of claim 10, in which said diffusion 40 step comprisesdiffusing Cr from a vapor phase in an oxidizing atmosphere.

References Cited UNITED STATES PATENTS 3,518,199 6/1970 Tsubouchi et al.252--62.9' 3,595,795 7/1971 Tsubouchi et al. 252-629 3,637,506 1/1972Tanizake et al. 25262.9

FOREIGN PATENTS 814,506 6/1969 Canada 252-629 1,926,812 2/1970 Germany252-629 OTHER REFERENCES Takanashi et al.: Japan J. Appl. Phys. 9(1970), p. 1006.

EARL C. THOMAS, Primary Examiner I. COOPER, Assistant Examiner

